The simplest type of bicycle rear wheel suspension is known as ‘single pivot’ suspension system. A single pivot suspension system comprises a chain stay or swing arm that pivots about a main pivot point near a bottom bracket or on a seat tube of a bike frame. A second pivot point may be located near a rear wheel axle and a shock absorber is normally connected to the top tube. A single pivot suspension is simple and lightweight, however usually suffers from the drawback of having a very progressive (rising rate) shock leverage ratio and excessive lateral flexure. The shock leverage ratio is defined as the ratio of rear wheel travel to shock travel. The shock leverage ratio may be constant throughout suspension travel (linear) or variable throughout suspension travel (progressive or regressive). A progressive (rising rate) shock leverage ratio means that the shock becomes harder to compress further into the suspension travel. A very progressive leverage ratio can mean that the full available travel of the suspension is not used.
Suspension designers have found that improved suspension performance is gained by a suspension system having a close to linear shock leverage ratio combined with a progressive air shock. Using an air shock instead of a coil spring shock reduces the weight of the suspension. For the simple single pivot design described above, because the suspension is usually very progressive, a coil spring must be used (which is usually linear) for reasonable suspension results, and this is accompanied by a weight penalty.
To obtain a more desirable shock leverage ratio, most modern bike rear wheel suspension systems use a rocker link mounted to the seat tube or top tube and pivotally connected to the shock. The rocker link can be designed to allow the shock to be connected to the bike's top tube, down tube, or orientated in a near vertical position connected to the main frame near the bottom bracket. An air shock can also usually be used with a rocker link design. An example of such a suspension system is a ‘linkage driven single pivot’ also known as ‘four bar’. A four bar suspension has a main pivot point positioned on the frame above the bottom bracket. A second set of pivot points are located on the seat stay near the rear axle. The seat stay is also pivotally connected to the rocker link A variation of this system is the ‘Horst link’ or ‘FSR’ design, whereby the second set of pivot points are located on the chain stays instead of the seat stays. Locating the second set of pivots at the rear wheel axle is known as a ‘Split pivot’ or ‘Active braking pivot’. While each of the above suspension systems can offer a more desirable shock leverage ratio compared to a simple single pivot, the disadvantage is that the rocker link adds weight, complexity and increases manufacturing cost.
Another variation of the above described suspension system is known as a ‘flex stay’. In a ‘flex stay’ system, the second rear set of pivots is eliminated and the rear triangle is designed to have some flex in the vertical plane to accommodate the changing angles of the rear triangle through suspension travel. A drawback of obtaining flex in the vertical plane is that invariably a degree of twist or flex in the horizontal direction will exist. Horizontal flex in the rear triangle is felt as slop, which is detrimental to bike handling, and increased flex can lead to material fatigue and shortened lifespan of the frame. Despite these drawbacks, the ‘flex stay’ system does eliminate a pair of pivot points which reduces the unsprung weight of the bike.
In addition to the shock leverage ratio, there are other suspension parameters that designers seek to optimise, including for example the rear wheel travel path. The ‘Virtual Pivot Point (VPP)’ and ‘DW link’ are two such systems that were developed in order to optimise the rear wheel travel path. By allowing the rear wheel to travel rearwards and upwards for the initial part of the shock movement, these designs can be more efficient at absorbing small bumps, particularly square edged bumps. This rear wheel travel path is achieved by having two sets of main pivot points somewhere near the bottom bracket. This creates a tighter radius for the rear wheel to rotate around. These designs usually feature the same number of pivot points as a Four Bar, Horst link, or Split pivot system. There is a disadvantage to these designs however, as pivot points gradually wear and develop a small amount of slop. This can allow a small amount of side to side play to be felt at the rear wheel. This amount of unwanted play can be measured at the rear wheel as a measure of the degree of slop in the pivot multiplied by the distance between the pivot point and the rear wheel axle. This problem obviously exists in every design, but with two sets of pivot points between the main frame and the rear wheel axle, the potential slop is nearly doubled.
It is against this background and the problems and difficulties associated therewith that the present invention has been developed.
Certain objects and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.